Silk fiber is a natural fiber having high biocompatibility. It is thin, strong, and has appropriate elasticity and softness. In addition, it exhibits a remarkable sliding property when spun into thread, and is easily manageable both in tying and untying. Therefore, silk fiber is often used as a surgical suture. Because silk is a protein in nature, silk suture is gradually decomposed and can be absorbed by the body even when stitches are left in the body. However, since the rate of degradation in vivo is slow, silk suture is usually categorized as a non-absorbable suture material, which is to be removed from the body. There are other types of sutures, such as regenerated collagen, which are degraded and absorbed in vivo, and stitches produced thereby do not have to be removed. However, since this type of suture has strength only one tenth that of silk suture, its resiliency is low and knot tying is not easy.
Production of regenerated silk thread from silk fiber of domesticated silkworms (hereinafter may be referred to simply as “domesticated silkworm silk fiber”) typically involves use of a solvent for dissolving silk fibroin originating from domesticated silkworms (hereinafter this fibroin may be referred to simply as “domesticated silkworm silk fibroin”), including an aqueous solution of a neutral salt (such as lithium bromide) or an aqueous solution of a complex salt (such as copper ethylenediamine). Since these aqueous solutions act to rupture strong hydrogen bonds in the β-sheet structure of silk fibroin, the following problems have been noted. That is, if the silk fibroin is left in the solvent over a long period of time, even molecular chains are ruptured thereby decreasing the molecular weight; salts tend to remain; and the resultant regenerated silk fiber exhibits poor mechanical properties.
Incidentally, hexafluoroisopropanol (HFIP) has been known as a solvent which prevents a decrease in molecular weight of silk fibroin and is useful for obtaining regenerated silk fiber having excellent mechanical properties (Patent Document 1). Specifically, silk fiber originating from natural domesticated silkworm is first dissolved in an aqueous salt solution such as lithium bromide solution, followed by desalting through dialysis and cast-drying, to thereby yield a silk fibroin film, and this film is dissolved in HFIP.    Patent Document 1: Japanese Unexamined Patent Application Publication (Translation of PCT application) No. 7-503288
Also, since HFIP requires a prolonged time for completing dissolution, there has recently been reported use of hexafluoroacetone (HFA), which is a compound analogous to HFIP, to produce regenerated silk thread (Patent Document 2).    Patent Document 2: JP-A-2004-68161
However, since the objective of the development of those regenerated silk fibers was natural silk fiber, efforts had been exclusively directed to obtain such regenerated silk fiber that approximates the strength and elongation of natural silk fiber, and thus, no study has heretofore been performed regarding in vivo degrading ability of fiber.
Under the above circumstances, the present inventors have conducted studies on the in vivo degradation of regenerated silk fiber produced by use of a fluorine-containing solution, and have found that, in the presence of a poteolytic enzyme, the fiber is more easily degraded than natural silk fiber, and that when hexafluoroisopropanol is used as a spinning solution and hematin is added thereto, tensile strength of the resultant thread can be improved.
Incidentally, black sutures are favorably used by surgeons in United States of America, because they are easier to see against the background of body tissue. In recent years, also in Japan, use of back sutures has become more popular. However, since the dying steps in manufacture of black sutures are more intricate than white sutures, there exists demand for more convenient method to impart black color to sutures.